Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id ; Mon, 15 Jul 2002 08:38:01 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id ; Mon, 15 Jul 2002 08:38:00 -0400 Received: from cmailg7.svr.pol.co.uk ([195.92.195.177]:7793 "EHLO cmailg7.svr.pol.co.uk") by vger.kernel.org with ESMTP id ; Mon, 15 Jul 2002 08:38:00 -0400 Date: Mon, 15 Jul 2002 13:40:35 +0100 To: Andrew Theurer Cc: linux-kernel@vger.kernel.org, linux-lvm@sistina.com Subject: Re: [Announce] device-mapper beta3 (fast snapshots) Message-ID: <20020715124035.GA4609@fib011235813.fsnet.co.uk> References: <3D2F6464.60908@us.ibm.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <3D2F6464.60908@us.ibm.com> User-Agent: Mutt/1.4i From: Joe Thornber Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1756 Lines: 40 Andrew, On Fri, Jul 12, 2002 at 06:21:08PM -0500, Andrew Theurer wrote: > Thanks for the results. I tried the same thing, but with the latest > release (beta 4) and I am not observing the same behavior. I've just read through the evms 1.1-pre4 code. You (or Kevin Corry rather) have obviously looked at the device-mapper snapshot code and tried to reimplement it. I think it might have been better if you'd actually used the relevent files from device-mapper such as the kcopyd daemon. Also I have serious concerns about the correctness of your implementation, for example: i) It is possible for the exception table to be updated *before* the copy on write of the data actually occurs. This means that briefly the on disk state of the snapshot is inconsistent. Users of EVMS that experience a machine failure will therefor have no option but to delete all snapshots. ii) The exception table is only written out every (EVMS_VSECTOR_SIZE/sizeof(u_int64_t)) exceptions. Surely I've misread the code here ? This would mean I could have a machine that triggers 1 fewer exceptions than this, then does nothing to this volume, at no point would the exception table get written to disk? Or do you periodically flush the exception table as well ? Again if the machine crashed the snapshot would be useless. iii) EVMS is writing the exception table data to the cow device asynchronously, you *cannot* do this without risking deadlocks with the VM system. - Joe - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/